Engine out cold-start emissions generated before light-off of an exhaust system catalytic converter may contribute a large percentage of the total exhaust emissions. To expedite the attainment of the catalyst light-off temperature, engine systems may inject air into the exhaust manifold to combust unburned fuel remaining in the exhaust. Additionally, or optionally, the injection of air may be supplemented with additional fuel to substantially increase the exhaust temperature and thereby decrease the light-off time.
One example of such an engine system is provided by Uhrich et al. in U.S. 2010/0263639. Herein, during an engine cold-start, an engine is operated with positive valve overlap while a turbocharger compressor is driven at least partially by a motor. In this way, a blow-through air flow is generated into the engine exhaust manifold through the cylinders of the engine. Fuel is injected with the blow-through air. The blow-through air exothermically reacts with the fuel in the exhaust and heats up the exhaust catalyst.
However, the inventors herein have recognized potential issues with such a system. As one example, the approach relies on a single fuel injection to heat the engine catalyst as well as attain the desired exhaust air-to-fuel ratio. Since the amount of heat directed to the catalyst is then adjusted with the blow-through air flow, the system may be heat limited. As another example, the approach uses a rich cylinder fuel injection in conjunction with the blow-through airflow to attain the desired air-to-fuel ratio in the exhaust mixture. However, during some engine cold-start conditions, a lean cylinder fuel injection may be desired (for example, to reduce exhaust NOx emissions). As such, the approach of Uhrich is incapable of heating the catalyst and providing a stoichiometric exhaust air-to-fuel ratio with a lean injection.
Thus, in one example, some of the above issues may be at least partly addressed by a method of operating a boosted engine. One example embodiment comprises, during an engine cold start, operating the engine with positive intake to exhaust valve overlap to drive a boosted blow-through airflow into an engine exhaust through engine cylinders. The method further comprises, injecting a first amount of fuel during the valve overlap, injecting a second amount of fuel outside of the valve overlap, and exothermically reacting the blow-through airflow with fuel in the exhaust.
In one example, a vehicle engine may include a turbocharger coupled between the engine intake and the engine exhaust. During an engine cold start, for example before a catalyst light-off temperature is attained, an engine may be operated with positive intake to exhaust valve overlap, while the turbocharger compressor is operated to drive a boosted blow-through airflow through the engine cylinders, into the exhaust manifold. During the valve overlap, a first amount of fuel may be injected into a cylinder, along with the blow-through airflow. A second amount of fuel may be injected and combusted in the same cylinder outside of the valve overlap. For example, the second amount of fuel may be injected after the valve overlap, but while still in the intake stroke of the same combustion cycle. Alternatively, the second amount of fuel may be injected outside of (e.g., before) the valve overlap, but while still in the intake stroke of the immediately preceding combustion cycle. In still other examples, the first and second amounts may be injected into different cylinders, the cylinders selected based on their firing order. For example, the selection of cylinders may allow the blow-through mixture and the cylinder combustion mixture to be generated at substantially the same time in the different cylinders and then mixed in the engine exhaust.
As such, the total amount of fuel injected (i.e., first and second injection amounts combined) may be adjusted to provide a final desired exhaust gas mixture air-to-fuel ratio (e.g., around stoichiometry). A split ratio of the first injection amount relative to the second injection amount in the total amount of fuel injected may be adjusted based on engine operating conditions, including an exhaust catalyst temperature, to provide a desired heat of oxidation. For example, when the exhaust catalyst is at a lower temperature, the first injection amount may be increased while the second injection amount is correspondingly decreased. The resultant rich blow-through air-fuel mixture can be mixed with the lean cylinder combustion mixture to generate a stoichiometric exhaust gas mixture wherein the rich blow-through air-fuel mixture increases the heat delivered to the exhaust catalyst while the lean cylinder combustion reduces cold-start exhaust NOx emission. In an alternate example, when the exhaust catalyst is at a higher temperature (but still below the light-off temperature), the first injection amount may be decreased, while the second injection amount is correspondingly increased. The resultant lean blow-through air-fuel mixture can be mixed with the rich cylinder combustion mixture to also generate a stoichiometric exhaust gas mixture wherein the lean blow-through air-fuel mixture decreases the heat delivered to the exhaust catalyst while the rich cylinder combustion is used to maintain engine torque and exhaust air-fuel ratio.
In this way, by injecting some fuel while boosted air is directed though the cylinders, fuel may be mixed thoroughly with blow-through air before reaching the catalyst. By combusting some fuel in an engine cylinder during a subsequent intake stroke, and mixing the cylinder combusted exhaust gas with the blow-through air-fuel mixture in the exhaust manifold, the resultant exhaust gas mixture can be used to expedite attainment of catalyst light-off conditions. Specifically, an exothermic reaction of the blow-through air-fuel mixture with the products of the cylinder combustion (including remaining unburned fuel, and burned fuel products such as short chain hydrocarbons (HCs) and carbon-monoxide (CO)) may be promoted to raise the temperature at the exhaust catalyst. By varying the relative amount of fuel in the two injections, the amount of oxidation heat directed to the catalyst can be varied while maintaining the exhaust mixture at stoichiometry. By rapidly increasing the catalyst temperature, the catalyst light-off time may be decreased and the quality of emissions may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.